engineering model. Connective tissue is some times referred to as being viscoelastic in nature. It contains both a viscous (permanent) deforma tion characteristic and an elastic ( or temporary) deformation characteristic. The two characteris tics combine to give connective tissue its unique qualities.II-IS This model incorporates a spring (elastic) and a hydraulic cylinder (plastic) linked in series to help depict this deformation quality (Figure
3-6).
The elastic component of connective tissue represents the temporary change in length when subjected to stretch (spring portion of model). The elastic component has a post-stretch recoil in which all length or extensibility gained during stretch or mobilization is lost over a short period of time (Figure
3-7).
In the elastic model, the spring recoils when tension or force is removed. The elastic component is not well understood but is believed to be the slack taken out of the connective tissue fibers. For example, a regular connective tissue has a loose basket weave con figuration of collagen f ibers. When a stretch is placed on the tissue, the slack is taken out as the fibers align themselves in the general direction of the stretch (Figure3-8).
When the stretch is removed, the f ibers assume their previous orientation and the change in length is lost.The viscous (or plastic) component repre sents the permanent deformation characteristic of connective tissue. After stretch or mobiliza tion, part of the length or extensibility gained remains even after a period of time (hydraulic cylinder portion of model). There is no post mobilization recoil in this component (Figure
3-9).
In the model, the hydraulic cylinder has been opened and does not close. Presumably, the permanent change results from breaking in termolecular and intramolecular bonds between collagen molecules, f ibers, and cross links.The viscoelastic model is then simply the vis cous and elastic portions of the model combined and arranged in series (Figure
3-10).
After a force is applied to the connective tissue through stretch or mobilization, a net change in length is achieved. Some of the change is quickly lost, while some remains.The combination of viscous and elastic prop erties allows for connective tissue to respond by creep and relaxation.lo Creep occurs when a load is applied to a tissue over a prolonged period of time, as in progressive stretching. This allows a gradual elongation of the tissue. The degree of deformation is more determined by the dura tion of force applied to the tissue rather than the amount of force. A lesser load over a greater period of time will produce a larger amount of
(A) (8) (C) (D) Collagen fibers + Ground substance matrix
Histology and Biomechanics o/Myofascia 3S
Tendons Ligaments Joint capsules Aponeuroses Fascia etc.
Viscous properties
---i�
Plastic stretchHydraulic cylinder model
Elastic properties ---l� Elastic stretch
Spring model force
Tensile force
Figure 3-6 (A) The primary and secondary organization of connective tissue in the body. (B) Schematic
representation of a viscous element in material capable of permanent (plastic) deformation. (C) Schematic
representation of an elastic element in material capable of recoverable (elastic) deformation. (D) A simplified
model of collagenous tissue. Connective tissue is a viscoelastic material: When stretched, it behaves as if it has
both viscous and elastic elements connected in series. Source: Reprinted with permission from The Physician
MODEL
1'R£ L.OOO "TENs.1..E
1
<'OSrL.CW>Figure 3-7 Schematic representation of the visco elastic model of elongation-elastic component in which no permanent elongation occurs after applica tion of tensile force. Source: Reprinted from Myofas cial Manipulation: Theory and Clinical Management (p 4) by A.1. Grodin and R. Cantu with permission of Forum Medicum Inc, ; 1989.
creep. An elevation in temperature will cause corresponding increases in creep. Hence, when stretching tight connective tissue, warmed tissue held for a sustained period will be more pliable than cold tissue stretched quickly.9,lo
If force is applied intermittently, as in progres sive stretching, a progressive elongation may be achieved. In Figure
3-11
A, strain, or percent elongation, is plotted against time for the pur poses of illustrating this phenomenon. Initially, there is a rapid elongation of the tissue, again representing the contribution of the elastic por tion of connective tissue. As time passes, less elongation is achieved, representing the con tribution of the viscous portion of connective tissue. When the stress is eventually released, the tissue immediately loses some of the previously attained elongation. Again, this phenomenon is consistent with the elastic characteristics of connective tissue. Not all the change in length is lost, however, because the tissue was stretched into the viscous or plastic range.If the stress is reapplied to the tissue, the curve looks identical, but starts from the new length achieved after the first stretch (Figure 3-11
B).
Again, the initial elongation is very rapid, but gradually slows as the tissue makes the transi tion from elasticity to plasticity. When the stress is re-released, another portion of the change in length is lost, and a portion is also retained. With each progressive stretch, the tissue has
some gain in total length that is considered per manent.
This phenomenon is seen often in the clinical setting. In stretching a restricted joint capsu Ie, for example, a certain increase in range of motion may be achieved during a particular treatment session. The patient may return a day or two later with a range of motion greater than the original range, but less than that achieved at the end of the previous treatment. In other words, some range is lost due to the elastic com ponent, and some is retained due to the plastic, or viscous, component.
Although the plastic component represents a permanent elongation, connective tissue is still capable of losing the elongation. The half life of collagen is
300
to500
days in mature\
B
Figure 3-8 Diagram showing the weave pattern of collagen, with A and B repr esenling elastic stretch and recoil of collagen fibers. Source: Reprinted from Donatelli R. and Owens-Burkhart, H., Effects of Im mobilization on Ihe Extensibility ofPeriarlicular Con nective Tissue, Journal of Orthopaedic and Sports Physical Therapy, Vol. 3, pp. 67-72, with permission of the Orthopaedic and Sports Sections of the Ameri can Physical Therapy Association.
Histology and Biomechanics of Myofascia 37
nontraumatized conditions.16 Over time, new collagen is laid down to replace older collagen. New collagen is laid down according to stresses (or lack of stresses) applied to the tissue. If the tissue is not stressed for long periods of time, it
I
A <---i "ME. B
Figure 3-9 Schematic representation of the viscoelastic model of elongation-plastic
component in which deformation remains
after the application of tensile force. Source:
Reprinted from Myofascial lvlanipulation. TheOl)' and Clinical Management (p 5) by
AJ. Grodin and R. Cantu with permission of Forum Medicllm Inc, © 1989.
Figure 3-10 Schematic representation of the viscoelastic model of elongation-some elon gation is lost and some is retained after the ap plication of tensile force. Source: Reprinted from Myofascial Manipulalion: Theory and Clinical Management (p 5) by A.J. Grodin and R. Cantu with permission of Forum Med icum Inc, © 1989.
will adaptively shorten as collagen is laid down in the context of the length of the tissues and
lack of stresses applied. Wolff's law, which states
that "bone adapts to the stresses applied,"7 can
be applied to connective tissue. All connective
r
< nt-'E
Figure 3-11 (A) Elongation of connective tissue (strain) plotted against time. (B) Repeated elongations of cOllnective tissue (strain) plotted against time. Source: Reprinted from Myofascial Manipulalion: Theory and Clinical Managemenl (pp 5-6) by A. F. Grodin and R. Cantu with permission of Forum Medicum Inc, © 1989.
tissue seeks metabolic homeostasis commen surate with the stresses being applied to that particular tissue. Wolff's law, however, applied to connective tissue, has a functional as well as a dysfunctional aspect. Abnormal stresses chroni cally applied to connective tissues may change the tissue resulting in dysfunction in the tissues and the adjacent structures supported by that tissue (i.e., facet joints, etc.). A clinical example of this phenomenon is the connective tissue band that develops in the patient with spondylolisthe sis. Because the spine in this condition cannot withstand the anterior shear forces applied daily, the body responds by laying down connective tissue, in time forming a connective tissue band. Normal stresses, or carefully controlled stresses (i.e., those stresses imparted externally by the clinician in the form of manipulation, or by the patient, in the form of exercises), may positively change the metabolic and physical homeostasis of the tissue. Collagen production is thus less haphazard, more organized, and laid down in a quantity and direction more suited to optimal tissue function. This concept is more fully de veloped in Chapter 4.
Specific Characteristics
Dense regular connective tissue. Ligaments and tendons are categorized as dense regular connective tissue. Dense parallel arrangement of collagen fibers characterizes dense regular con nective tissue (Figure 3-12). The high propor tion of collagen to ground substance and the parallel arrangement of the fibers accounts for the high tensile strength and limited extensi bility of these tissues. Because of the histol ologic makeup of these tissues, they are the least responsive to manual work. Because of the compactness and density of collagen fibers and the relatively small proportions of ground substance, the tissue is not highly metabolic, and not very vascular, accounting for the increased healing time required after trauma.
The primary function of tendon is to attach muscle fibers to bone and to transmit forces expended by muscle to the bone with limited elongation, allowing for tension or joint move-
Figure 3-12 Drawing of dense regular connective tissue, showing the parallel arrangement of collagen
fibers. Source: Reprinted from Gray s Ana/omy, ed 35
(p 40) by P. Williams and R. Warwick with permission
ofW.B. Saunders, © 1973.
ment.17,18 The collagen fibers in tendon have, therefore, been designed in a parallel arrange ment to provide the highest unidirectional tensile strength possible. The stress-strain relationship of tendon is similar to that of other connective tissues, with some minor differences. When a tendon is stressed, the toe region (elastic com ponent) of the stress-strain curve is generally smaller due to the parallel arrangement of col lagen fibers. This indicates less realignment of fibers than found in other connective tissues during tension. The toe region is generally fol lowed by a moderately linear region with a slightly greater slope, which is indicative of the tendon's greater stiffness. With further tensile deformation, small dips or hitches appear in the curve that possibly represent early tissue microfailure. Finally, with further loading, the tissue fails completely, and the stress-strain curve drops to zero. 17.19
The primary function of ligament is to check excessive motion in joints and to guide joint motion.17,18 Ligaments have a less consistent parallel arrangement of collagen fibers than does tendon (Figure 3_13).20 Under light mi croscopy, the orientation of the collagen takes on an undulating configuration known as "crimp. "21
39
Figure 3-13 Drawing of ligamentous tissue, showing overall parallel arrangement of fibers, but somewhat less parallel than tenelon. Source: Reprinteel from Grays Anatomy, eel 35 (p 40) by P. Williams anel R. Warwick with permission ofWB. Saunders, © 1973.
This crimp phenomenon is thought to be respon sible for the mildly elastic characteristics ofliga ment. The ligament functions biomechanically as a spring, until all of the crimp is straightened out and, subsequently, becomes more tensile when the collagen fibers are actually stressed. The ultimate biomechanical result is that liga ments have somewhat less tensile strength per unit area than tendon, but have slightly more yield (Table 3-2).
Histology and Biomechanics of My of asci a
Dense irregular connective tissue. Dense ir regular connective tissue includes, but is not limited to, joint capsules, aponeuroses, penos teum, and fascial sheaths under high degrees of mechanical stress. The major difference between dense irregular and dense regular connective tissue is the orientation of collagen fibers. [n dense irregular connective tissue, the collagen fibers are aligned multidirectionally in order to withstand multidirectional stresses (Figure 3-14). The lumbodorsal fascia, for example, has many different attachments, and is pulled in different directions during the spine's normal function.
Loose irreguillr connective tissue. Loose ir regular connective tissue includes, but is not limited to, the superficial and some deep fascia, as well as muscle and nerve sheaths. The sup portive framework of the lymph system and the internal organs is also classified as loose ir regular connective tissue. Loose irregular con nective tissue is generally characterized by a sparse, multidirectional framework of collagen and elastin. Loose irregular connective tissue contains a greater amount of ground substance per unit area than other types of connective tis sues. Because of sparse concentrations of col lagen in this type of tissue, loose irregular con nective tissue is the most elastic and typically has the greatest potential for change when ma nipulated by external forces.
Table 3-2 Classification of Connective Tissue
Tissue Type Specific Structures Characteristics of the Tissue
Dense regular Ligaments, tendons
Dense irregular
Loose irregular
Aponeurosis, periosteum, joint capsules, dermis of skin, areas of high mechanical stress Superficial fascial sheaths, muscle
and nerve sheaths, support sheaths of internal organs
Dense, parallel arrangement of collagen fibers; proportionally less ground substance
Dense, multidirectional arrangement of collagen fibers; able to resist multidirectional stress
Sparse, multidirectional arrangement of collagen fibers; greater amounts of elastin present
Figure 3-14 Drawing of dense irregular cOlUlective tissue, showing the multidirectionality as well as high density of collagen fibers. Source: Reprinted from Grays Anatomy, ed 35 (p 40) by P. Williams and R. Warwick with permission ofW.B. Saunders, © 1973.
HISTOLOGY AND BIOMECHANICS OF MUSCLE
As previously stated, the myofascial tissues account for the majority of tissue being affected by orthopedic manual therapy. A large portion of the myofascial tissues includes muscle tissue. As with connective tissue, a basic understanding of muscle tissue is also essential for an appropri ate empirical understanding of myofascial ma nipulation. Knowledge of trauma, immobiliza tion, and remobilization of muscle tissue must be built based on the scientific principles that will be outlined as follows. The histology and physi ology of muscle tissue alone occupies whole chapters in textbooks. The purpose of this sec tion is to provide a basic overview of muscle histology and how it relates to connective tissue. Much of the knowledge of mammalian skeletal muscle comes from studies of frog skeletal muscle, which is anatomically and histologically similar.
Histology
Muscle is histologically categorized into three types: skeletal, smooth, and cardiac. This section focuses primarily on skeletal muscle, which in turn will provide a basis for understanding car diac and smooth muscle types. Skeletal, or stri ated muscle, is so named because of its striated or banded appearance under light microscopy. The striations reflect the functional contracti Ie unit of the muscle called the sarcomere. Muscle is also functionally characterized by fiber type based on speed of contraction or relaxation, bio chemistry and metabolism, and in circulation. Mechanism of Growth ill Skeletal
Muscle
The total number of actual muscle fibers in a muscle is reached sometime before birth. Lon gitudinal growth in a muscle is accomplished in early years by an increase in the length of the individual sarcomeres and by addition of sarco meres. Increases in diameter are accomplished by the addition of myofilaments in parallel ar rangement. Likewise, the muscle shortens by losing sarcomeres and decreases in diameter by losing myofilaments. With prolonged disuse, the muscle fibers degenerate and the tissue is re placed with less metabolically active connective tissue. Human skeletal muscle, however, does have some limited regeneration potential. Satel lite cells, which are believed to be a persisting version of the prenatal myotubes found inside basement membranes, can become activated to produce a limited amount of new muscle fibers. The number of new fibers that can be produced, however, cannot compensate for the amount lost during major muscle trauma or degeneration. Cellular alld Histological Organizatiol1
of Skeletal Muscle
The contractile proteins of striated muscle are actin and myosin. The actin and myosin in teract in a ratchet-type manner to shorten the muscle (Figure 3-15). Actin and myosin fila ments are contained in the functional contrac tile unit of muscle called the sarcomere. The